Fire sprinklers can be automatic or open orifice. Automatic fire sprinklers operate at a predetermined temperature, utilizing a fusible link, a portion of which melts, or a frangible glass bulb containing liquid which breaks the bulb at high temperatures. The water stream impacts a deflector, which produces a specific spray pattern, designed in support of the goals of the sprinkler type (i.e., control or suppression). Modern sprinkler heads are designed to direct spray downwards. Spray nozzles are available to provide spray in various directions and patterns. The majority of automatic fire sprinklers operate individually in a fire. Contrary to what is often shown in movies, the entire sprinkler system does not activate, unless the system is a special deluge type.
Open orifice sprinklers are only used in water spray systems or deluge sprinklers systems. They are identical to the automatic sprinkler on which they are based, with the heat sensitive operating element removed.
Automatic fire sprinklers utilizing frangible bulbs follow a standardized color-coding convention indicating their operating temperature. Activation temperatures correspond to the type of hazard against which the sprinkler system protects. Residential occupancies are provided with a special type of fast response sprinkler with the unique goal of life safety.
Maximum CeilingTemperatureTemperatureColor Code (withGlass BulbTemperatureRatingClassificationFusible Link)Color100° F./38° C.135-170° F./57-77° C. OrdinaryUncolored orOrange (135°) orBlackRed (155°)150° F./66° C.175-225° F./79-107° C. IntermediateWhiteYellow (175°) orGreen (200°)225° F./107° C.250-300° F./121-149° C.HighBlueBlue300° F./149° C.325-375° F./163-191° C.Extra HighRedPurple375° F./191° C.400-475° F./204-246° C.Very Extra HighGreenBlack475° F./246° C.500-575° F./260-302° C.Ultra HighOrangeBlack625° F./329° C.650° F./343° C.Ultra HighOrangeBlack
Most sprinkler systems installed today are designed using an area and density approach. First the building use and building contents are analyzed to determine the level of fire hazard. Usually buildings are classified as light hazard, ordinary hazard group 1, ordinary hazard group 2, extra hazard group 1, or extra hazard group 2. After determining the hazard classification, a design area and density can be determined by referencing tables in the National Fire Protection Association (NFPA) standards. The design area is a theoretical area of the building representing the worst-case area where a fire could burn. The design density is a measurement of how much water per square foot of floor area should be applied to the design area. For example, in an office building classified as light hazard, a typical design area would be 1500 square feet and the design density would be 0.1 gallons per minute per square foot or a minimum of 150 gallons per minute applied over the 1500 square foot design area. Another example would be a manufacturing facility classified as ordinary hazard group 2 where a typical design area would be 1500 square feet and the design density would be 0.2 gallons per minute per square foot or a minimum of 300 gallons per minute applied over the 1500 square foot design area.
After the design area and density have been determined, calculations are performed to prove that the system can deliver the required amount of water over the required design area. These calculations account for all of the pressure that is lost or gained between the water supply source and the sprinklers that would operate in the design area. This includes pressure losses due to friction inside the piping and losses or gains due to differences in elevation between the source and the discharging sprinklers. Sometimes momentum pressure from water velocity inside the piping is also calculated. Typically these calculations are performed using computer software but before the advent of computer systems these sometimes complicated calculations were performed by hand. This skill of calculating sprinkler systems by hand is still required training for a sprinkler system design Technologist who seeks senior level certification from engineering certification organizations such as the National Institute for Certification in Engineering Technologies (NICET).
Sprinkler systems in residential structures are becoming more common as the cost of such systems becomes more practical and the benefits become more obvious. Residential sprinkler systems usually fall under a residential classification separate from the commercial classifications mentioned above. A commercial sprinkler system is designed to protect the structure and the occupants from a fire. Most residential sprinkler systems are primarily designed to suppress a fire in such a way to allow for the safe escape of the building occupants. While these systems will often also protect the structure from major fire damage, this is a secondary consideration. In residential structures sprinklers are often omitted from closets, bathrooms, balconies, garages and attics because a fire in these areas would not usually impact the occupant's escape route.
If water damage or water volume is of particular concern, a technique called Water Mist Fire Suppression may be an alternative. This technology has been under development for over 50 years. It hasn't entered general use, but is gaining some acceptance on ships and in a few residential applications. Mist suppression systems work by lowering the temperature of a burning area through evaporation rather than “soaking”. As such, they may be designed to only slow the spread of a fire and not extinguish it. Some tests that may or may not be biased, showed the cost of resulting fire and water damage with such a system installed to be dramatically less than conventional sprinkler systems.
The commercial market demands regarding glass bulbs for sprinklers for automatic fire extinguisher systems and also for other thermal release means, are for much shorter release times, which may be up to almost ten times shorter. Such shorter release times must be achieved without sacrificing durability of the glass drum or the axial loading in the sprinkler.
One prior proposal to meet these requirements consisted of reducing the volume of breaking liquid in the glass bulb by placing a solid displacement member in the bulb without modifying the dimensions of the glass body, and therefore without modifying the strength characteristics. See U.K. Patent No. 2,120,934, published Dec. 14, 1983. Attempts have also been made to reduce the release times by reducing the overall diameter of glass drum so as to bring about a more favorable ratio of the surface area to the volume of the bulb, and consequently of the volume of the breaking liquid in the bulb. However, these attempts have lead to an unacceptable reduction in strength.
In sprinklers, which constitute the main field of use for glass thermo bulbs, such bulbs act as a thermally active release member to keep a valve closed. The elongate bulb is generally secured at its ends between two ends of the sprinkler and the ends apply an axial force on the ends of bulb. In the case of a fire, the glass bulb shatters and allows the valve to open and to release the fire extinguishing medium, which is usually water.
Such a glass bulb typically comprises a hollow and generally cylindrical or barrel shaped enclose or shaft, the length of which may vary widely. The bulb is often provided with an annular offset or shoulder in the wall at one end of the shaft so as to form the thermally active part together with the expansible breaking fluid or liquid confined within the glass enclosure. At the ends, which engage sprinkler abutments, flat, conical or curved, and substantially thermally inactive ends bound the shaft. One of the ends is normally referred to as the tip end, which is thin and tapered to a rounded point. The expansible breaking fluid is introduced into the bulb through the tip end during manufacturing, and thereafter the tip end is closed.
The glass bulb must be able to take a specific permanent load which is dependent upon the nature of the valve construction or release mechanism in the sprinkler as to insure that the sprinkler remains closed over several decades and is always kept in a state of readiness.
The Response Time Index is a calculated value taking into account the actual activating time of a glass bulb mounted in a sprinkler or other devices in given standard conditions. Fast response times are associated with lower RTI values.
                    RTI        =                                            (                                                -                                      t                    r                                                  ⁢                                  u                                            )                        ⁢                          (                              1                +                                  C                                      u                                                              )                                            ln            [                          1              -                              [                                                                            (                                                                        T                          ea                                                -                                                  T                          u                                                                    )                                        ⁢                                          (                                              1                        +                                                  C                                                      u                                                                                              )                                                                                                  T                      g                                        -                                          T                      u                                                                      ]                                      ]                                              (        1        )            RTI=Response Time Index [(ms)1/2]tr=actual response time of thermal release element (s)u=actual gas velocity in the test section of the wind tunnel (m/s)Tea=mean liquid bath operating temperature of sensitive detector element (° C.)Tg=actual gas temperature in test section (° C.)Tu=ambient air temperature during testing (° C.)C=Conductivity Factor [(m/s)1/2]UL Conditions: 135° C. at 2.54 m/s
Thermo bulbs with response times slower than an RTI value of 80 is used in all products requiring Standard Response functional properties as defined by local agencies or authorities in the USA, Europe and Asia and as specified in International Standard ISO 6182:1.
These types of thermo bulbs are used applications where Insurers Hazard Classifications require sprinklers, which have an RTI<80, e.g., as per LPC's attachment to BS 5306:2, TB 20: Selection of Sprinkler Heads, in the UK and for Concealed or Recessed type sprinklers. Other international regulations also require Intermediate Response bulbs.
These bulbs are specified for domestic sprinklers in the USA and where Insurers Hazard Classifications require Fast Response—or in Extended Coverage Sprinklers, which require faster operational times due to the increased distance between installed sprinklers.
The Super Fast and Ultra Fast bulbs F2.5, F2 and F1.5 are typically used in high performance products where very early activation is essential. Examples are ESFR Sprinklers or Water Mist products.
Previously known glass bulbs, which satisfy the appropriate standards imposed by insurance or governmental agencies, generally have a diameter between 8 and 12 mm, a wall thickness of 1 to 1.5 mm, and an overall length of 20 to 30 mm. Such relatively thick glass bulbs do not respond quickly to heat from a fire, and have rather long release times, i.e., the time lapse from the first occurrence of critical temperature to be sensed to the shattering of the bulb and release of the valve. Such long release times are a result of the unfavorable ratio of the heat-absorbing surface of the bulb to the volume within the bulb to be heated. U.S. Pat. No. 4,796,710 (JOB® GmbH) discloses a bulb with a unique bone shape design that uses reinforced ends to absorb loads from the mounting supports and to introduce these axially into a shaft of reduced diameter thus avoiding unfavorable shearing and bending stresses in the glass. The bone shape design allows for a low mass structure, which, combined with the special filling liquid, provides very short response time. But it is expensive to manufacture with the cost of the bulb approximately 40-50% of the total cost of a sprinkler head. It is also fragile and requires careful packaging to avoid damage during shipping and installation.